1. Technical Field
The present invention relates to an optical disk apparatus, and more particularly to optimization of recording power during data recording operation.
2. Related Art
An optical disk apparatus capable of recording data, such as a CD drive, a DVD drive, or the like, has hitherto optimized recording power by means of OPC (Optimum Power Control) and ROPC (Running Optimum Power Control), to thereby record data. OPC is processing for writing test data, for trial purposes, in a predetermined area on an optical disk at various recording power levels; and computing recording power at which the quality of a reproduced signal, such as a β value, the degree of modulation, and the like, acquired by reproduction of the test data, attains a target value. ROPC is processing for controlling recording power in an increasing or decreasing manner according to the quantity of reflected light acquired during recording of data (i.e., the quantity of reflected light acquired when the quantity of reflected light becomes stable after formation of pits, and is hereinafter called a “level B”) in light of a change in recording sensitivity attributable to a change in the geometry of a laser beam induced by unevenness in a coating of a recording film in the optical disk, the inclination of the optical disk, or the like. FIG. 7 shows changes in the quantity of reflected light acquired when a pit 100 is formed by irradiating a laser beam of recording power onto an optical disk. The quantity of reflected light is large at an early stage of formation of the pit 100. However, in time the thus-created pit causes diffraction to thus decrease the quantity of reflected light, and the quantity of reflected light becomes stable at a given value (a level B). The level B is detected by means of sample-holding a reflected light signal at this timing. The quantity of reflected light B is defined as the quantity of light reflected at a timing when the quantity of light reflected by the created pit has become stable at a given value.
In general, during ROPC, the quantity of reflected light Bo acquired at recording power Po determined through OPC (the quantity of reflected light acquired at timing when the quantity of light reflected by a pit has become stable at a constant value when the pit has been created at recording power Po) is stored, and recording power is determined such that a value β becomes constant. Consequently, recording power P is determined such that a relationship of Bo/Pon=B/Pn . . . (1) stands at initial recording power Po determined through OPC and the quantity of initial reflected light Bo. As described in Japanese Patent Laid-Open Publication No. 2003-263740, “n” is usually set to two or thereabouts. However, as described in Japanese Patent Laid-Open Publication No. 2003-248929, there may be a case where “n” is set in a range from 1.5 to 10; particularly, in the neighborhood of 3.2.
When the value of a parameter “n” is increased, ROPC becomes less effective. Accordingly, decreasing the value of “n” is conceivable. However, when the value of “n” is decreased, there may also arise a case where recording power fluctuates greatly to thus become divergent.
FIG. 8 shows hourly variations in recording power induced by ROPC when “n” is set to a small value (“n”≦2). During execution of ROPC, recording power P is determined on the basis of the quantity of current reflected light B in accordance with Equation (1). Accordingly, recording power fluctuates greatly depending on the quantity of reflected light B. When the quantity of current reflected light B is large, power which is greater than the current recording power is qualitatively required as recording power P. When pits are created at great recording power pursuant to this requirement, the degree of formation of pits becomes excessive, and the quantity of reflected light B is greatly decreased. For this reason, in accordance with a decrease in the quantity of reflected light B, power which is smaller than the current recording power is required as recording power P. When pits are created at small recording power pursuant to the requirement, the degree of formation of pits becomes insufficient. As a result of repetition of the above processing operations, the recording power P fluctuates as shown in FIG. 8, to thus fail to converge. Variations eventually arise in the degree of formation of pits, which contributes to a decrease in recording quality.